Microstructures having an internal cavity can be formed by making an assembly of two chips or two wafers or a chip-on-wafer with a spacer in-between. Such structures should have hermetically sealed cavities filled with a controlled ambient (gas composition and/or pressure).
These structures can be used for many different applications such as microaccelerometers, microgyroscopes, microtubes, vibration microsensors, micromirrors, micromechanical resonators or "resonant strain gauges", micromechanical filters, microswitches and microrelays.
Traditionally, for these applications, the ambient of the cavity is defined during the assembly of the several components by anodic, fusion or eutectic wafer bonding, wafer bonding using low temperature glasses or polymers as the brazing material and reactive sealing techniques.
A common drawback of these techniques is that they are rather limited in applicability, since device separation is difficult (the device has been made on one of the two wafers). It is also difficult to create electrical contacts. The drawbacks of three of the most common techniques are discussed herebelow.
The technique of diffusion bonding of a Si cap wafer on the device wafer requires flat Si surfaces and a high temperature process.
Wafer bonding techniques such as anodic bonding and silicon fusion bonding require a very clean environment, i.e., low particle contamination. There are applications that are not compatible with these boundary conditions of temperature and flatness. Furthermore, the technique of anodic bounding also requires flat surfaces and needs the application of a high voltage in order to achieve the bonding.
Finally, the technique of gluing does not provide a real hermetic bond.
U.S. Pat. No. 5,296,408 describes a fabrication method of making a microstructure having a vacuum sealed cavity therein, including the process steps of forming an aluminum filled cavity in a body of silicon material and heating the structure such that the aluminum is absorbed into the silicon material leaving a vacuum in the cavity. In one embodiment, a cavity is etched into a silicon wafer and filled with aluminum. A silicon dioxide layer is formed over the aluminum filled cavity and the structure is heated to produce the vacuum cavity.
The document "Fluxless flip-chip technology" by Patrice Caillat and Gerard Nicolas of LETI, published at the First International Flip-Chip Symposium, San Jose, Calif., February 1994 describes a flip-chip assembly of two chips with a solder sealing ring defining a cavity during the assembly itself. The assembly and the subsequent sealing are normally done in air or under an N.sub.2 purge. Similar conditions may exists for the other wafer bonding techniques as mentioned hereabove (except for the technique of reactive sealing).
Advantages of the Present Invention
The present invention is directed to a microstructure product and a method of fabricating a microstructure having an internal cavity. Preferably, the covity is sealed with a controlled ambient allowing a free choice of the sealing gas composition and the sealing pressure or vacuum.
The method preferably does not require special equipment to perform the fabrication of such microstructures in a vacuum or controlled inert gas ambient.
The method is suited for micro-electromechanical systems (MEMS) packaging wherein all the process steps are compatible with packaging equipment.